Adding a field to pass additional flags to the git whatsnew command

It's useful to be able to pass more flags down to the `whatsnew` command that generates the changelog.
For example, we do a lot of merging, and we're not interested in what is being merged, but rather that there was a merge. The `git whatsnew` command by default hides the actual merge commit and shows all the commits that have been merged. So now we can pass in `--first-parent -m` to hide all the commits being merged and show the actual merge commit.
The config is in the Git SCM config under the second "Advanced" section.